As is well known in the art, an AWD (All Wheel Drive) vehicle can be provided with at least one hydraulic disc coupling for distributing the driving torque from the vehicle engine to all of the vehicle wheels. Especially, such a coupling may be provided in the drive line between the front axle and the wheels of the rear axle, most often in the vicinity of the rear axle differential.
Sometimes, it may be desirable to use the AWD vehicle in a FWD (Forward Wheel Drive) mode. In this case the coupling can be disconnected, i. e. its discs are separated from each other.
By operating the vehicle with the coupling in a disconnect mode, rotating masses in the vehicle are reduced, leading to a lower fuel consumption.
In a practical case a propeller shaft or intermediate shaft of the vehicle, possibly including one or two bevel gear transmissions, has a hydraulic disc coupling at one end and a simple coupling, for example a dog clutch, at the other end. In the disconnect mode both these couplings must be disconnected for obtaining the desired effect.
When the AWD mode is resumed, the hydraulic disc coupling must resume its connected state (connect mode) very quickly, say within a period of 0.4-0.5 s or less. For several reasons such a short time from the fully disconnected state is difficult to obtain.
In order to secure proper disconnection, the play between neighboring discs in the coupling must be in the order of typically 0.08-0.10 mm, so that the drag torque from the coupling is minimized. This means that the required movement or stroke of a hydraulically operated piston of the coupling has to be considerable, as there are several discs in the coupling.
On the other hand, wear of the discs in the coupling will result in an even longer piston stroke and accordingly a longer time to reach the connect mode for the coupling from the disconnect mode.
The main object of the invention is thus to minimize the connect time for the coupling, even with worn discs in the coupling.